An arrhythmia is a disturbance in the normal rate, rhythm or conduction of the heartbeat. Arrhythmia may originate in an atrium or a ventricle. Atrial tachycardia (AT) and ventricular tachycardia (VT) (collectively referred to as tachycardia), are forms of arrhythmia in which the atria or ventricles contract at a high rate, e.g., 100 or more beats per minute. Atrial fibrillation (AF) and ventricular fibrillation (VF) (collectively referred to as fibrillation) are other forms of arrhythmia, characterized by a chaotic and turbulent activation of atrial or ventricle wall tissue. The number of depolarizations per minute during fibrillation can exceed 400. In addition, the fibrillation stimuli can occur in the refractory period of the surrounding myocardium.
Tachycardia can lead to fibrillation, which in turn can be life threatening. Tachycardia is also associated with other low cardiac output symptoms, such as fatigue. Many tachycardias are episodic, marked by abrupt onset but also abrupt termination. Still, tachycardia can cause considerable patient distress. Moreover, if untreated, tachycardia can lead to other dangerous life-threatening conditions, such as the development of blood clots which can cause stroke and possibly death.
Treatment for tachycardia may include anti-tachycardia pacing (ATP) or cardioversion, in which a train of high rate pulses or one or more high energy pulses is delivered to the heart in an attempt to restore a more normal rhythm. ATP is typically effective in converting stable tachycardias to normal sinus rhythm, and is often delivered via an implanted device. In many cases, a sequence of increasingly aggressive ATP therapies are applied until an episode of tachycardia is terminated. The implanted device can be configured to discontinue ATP and immediately apply cardioversion in the event the tachycardia degrades into fibrillation.
For some tachycardia episodes, existing ATP techniques may not be completely effective. For example, tachycardia may originate in a very localized site within a specific heart chamber. In that case, existing ATP techniques may apply therapy at locations or times that are not effective to end the tachycardia condition. Accordingly, there is a need for improved ATP therapy. Table 1 below lists a number of documents that disclose implantable devices designed to deliver ATP.
TABLE 1U.S. Pat. No.InventorIssue Date3,937,226FunkeFeb. 10, 19764,088,140Rockland et al.May 09, 19785,683,429MehraNov. 04, 19974,354,497KahnOct. 19, 19824,928,688MowerMay 29, 19905,873,896IdekerFeb. 23, 19996,078,837Peterson et al.Jun. 20, 20005,620,468Mongeon et al.Apr. 15, 19975,674,251Combs et al.Oct. 07, 1997
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth below, the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the techniques of the present invention.